1. Field of the Invention
The invention relates to a control ring for a stage of pivotable vanes in a turbomachine and means for guiding the ring during rotation of the ring to pivot the vanes.
Modern turbomachines often include stator vanes which can pivot around axles connecting them to the stator so that their so-called pitch angle may be adjusted in order to maintain satisfactory gas flow characteristics in all operating regimes of the machine. The pivot axles of the vanes are connected to links which are pivotally connected at their other end to a common ring, and rotation of the ring causes pivoting of the links, and hence the axles and the vanes.
2. Summary of the Prior Art
Research aimed at improving the efficiency of turbomachines has led also to improving the precision of the pitch angle of the vanes. To achieve this, it is necessary to ensure excellent coaxiality between the ring and a circular surface of the machine, such as the stator casing. For this purpose the ring is provided with shoes which are connected to the ring by mounting means so that they surround reference surfaces with a very small clearance, the mounting means permitting adjustment of the shoes in a radial direction.
An example of such an arrangement is disclosed in European Patent 0 375 593, wherein the mounting means for each shoe comprises a threaded bush crimped through the ring, and a shoe-holder having a guide rod which is slidable without rotation in a hole of non-circular section so as to enable the shoe to slide without being able to rotate. The shoe-holder is completed by a threaded rod which passes right through the ring, and a threaded sleeve is disposed around the rod by screwing the sleeve into the bush. The rotation of the sleeve in the bush determines the position of the shoe by pushing back the guide rod, and a lock-nut may be engaged with the thread of the rod protruding from the ring to compress the sleeve against the guide section.
It is an object of the invention to provide the ring with shoe mounting means which allow much finer setting of the radial positions of the shoes than the known arrangement described above.